The amplification of a brief pulse in a laser chain causes various distortions in the wave packet. The concern here is with the particular distortions associated with the use of lenses for propagating the laser beam. Lenses are optical components normally used in a laser chain, for example for increasing the size of the beam, for filtering the undesirable spatial frequencies and for transporting the image of a plane. In the majority of cases, focal systems consisting of two lenses are used.
An additional difficulty arises when the diameter of the laser beam becomes great. In this case, the differences in thickness of glass found from the centre to the edge of a lens creates a particular distortion relating to a propagation delay term between the central ray and the rays at the periphery of the beam. The main effect of this distortion is to delay the light energy of the centre of the wave packet compared with the energy at the periphery.
In high-energy so-called femtosecond chains, this delay is often of the same order of magnitude as the duration of the pulse. A femtosecond chain delivers light pulses of a few tens of femtoseconds (1 fs=10−15s) and an energy from a few joules for lasers of 100 terawatts) to a few tens of joules (for petawatt lasers). The problem with femtosecond chains is that, because of the delay, the physical target that in the end receives the energy of the wave packet focused at the end of the chain is illuminated for a time greater than the duration of the pulse. Consequently the instantaneous heating of the target is rapidly less effective and the peak intensity of the confinement, expressed in watts per cm2, is no longer optimized. It is therefore important to be able to correct the delay appearing in these laser chains in order to obtain brief-energy pulses with the minimum of defects.
It was in 1988 that the spatio-temporal effects created by the use of lenses was calculated for the first time by Bor (Z. Bor, Distortion of femtosecond laser pulses in lenses, Optics Letters, vol. 14, N° 2, 1989, pp 119-121). These effects are broken down into a group speed dispersion term and a propagation delay term (pupil delay). The distortions caused on the light pulse by the pupil delay are preponderant not only in laser chains but also in microscopy systems in which large-aperture lenses are necessary and in ultra-brief laser characterization instrumentation (such as autocorrelators and spiders using lenses). Bor demonstrated that the effect of pupil delay on the temporal broadening of the pulse could be 2 to 3 times greater than the temporal broadening caused by the dispersion of group speeds.
A light pulse can be described as being a wave packet consisting of a carrier 1, which oscillates at the frequency of the wave, multiplied by an envelope function 2. A light pulse is illustrated in FIG. 1.
The carrier and envelope do not propagate at the same speed in an optical medium. This is because the carrier moves at the phase speed vφ of the wave and the envelope moves at the group speed vg. There are thus, in a dispersive medium of index n, the following equations:vφ=λ.ν=c/n0 andvg=vφ−λ.(dvφ/dλ)λ
where λ is the mean wave length in the medium, ν the frequency of the wave, c the speed of light in a vacuum and n0 the index at the mean wavelength λ0 in a vacuum (λ=λ0/n0).
It should be noted that 1/vg can be written as a function of the parameters of the medium and of the mean wave length λ0:1/vg=n0/c−nλ.λ0/c where nλ=(dn/dλ)λ0 
The beam propagating the incident pulse is represented in FIG. 2 by five light rays. The light beam here passes through a convergent lens and the five incident rays converge at the focus F of the lens.
As the lens is convergent, the two peripheral rays pass through a small thickness of glass and reach the focus F first. The central ray, which passes through the greatest thickness of glass, arrives at the focus F with a delay δ. The two middle rays arrive at the focus with an intermediate delay.
We observe that the phase front is represented upstream of the lens by the plane P and downstream by the arc of a circle C, the centre of which is situated at the focus F. The phase of the wave is therefore conveyed as far as the focus F of the lens without deformation whereas the energy transported by the pulse, represented by the arc of a circle E, has a substantial distortion with respect to the phase front.
We will note here that, for a divergent lens, the delay δ would be reversed and the central ray would here this time be in advance with respect to the other rays.
The pupil delay δ is calculated from the following formula:δ=β.T.(h2/2f)
It will be explained how this formula is obtained.
For this purpose and in order to avoid complicated calculations, FIG. 3 presents a thin plano-convex lens, the orientation of which with respect to the beam is chosen solely in order to clarify the presentation. In this FIG. R is the radius of curvature of the convex face of the lens and n0 represents the index of the medium at the mean light wave length.
According to Fermat's principle, the optical path actually followed by the light between two points is stationary. This means that all the optical paths of the rays starting from the equi-phase plane P and going to the focus F are identical. We therefore necessarily have:
                    HF        =                ⁢                  OS          +          SF                                        =                ⁢                                            n              0                        ·            x                    +          SF                                        =                ⁢                                            n              0                        ·            x                    +          f                    
where f represents the focal distance of the lens.
For this thin lens, we have the following equations:R=(n0−1).f andx=h2/2R 
The pupil delay between the peripheral rays and the central ray is therefore as follows:
                    δ        =                ⁢                                            (                              HF                -                f                            )                        /            c                    -                                          ⁢                      (                          x              /                              v                g                                      )                                                  =                ⁢                              (                                          n                0                            ·                              x                /                c                                      )                    ⁢                                          -                      x            ·                          (                                                                    n                    0                                    /                  c                                -                                                                  ⁢                                                      n                    λ                                    ·                                                            λ                      0                                        /                    c                                                              )                                                              =                ⁢                                            (                                                n                  λ                                /                                  (                                                            n                      0                                        -                    1                                    )                                            )                        ·                          λ              0                        ·                                          h                2                            /              2                                ⁢                      c            ·            f                              
By putting β=nλ/(n0−1) (dispersive term) and T=λ0/c (the period of the wave), the expression of δ is then:δ=β.T.(h2/2f)
It can be noted that the pupil delay δ has an axial symmetry. This is because all the peripheral rays propagating on an aperture of radius h have the same delay δ with respect to the central radius.
In a convergent lens, the pupil delay δ is negative because of the dispersion term β. The delay δ in a convergent lens is then a temporal advance.
Conversely, the pupil delay δ will be positive in a divergent lens and the peripheral rays will have a real delay with respect to the central ray.
We will note that the total temporal delay δT of a chain is the sum of all the algebraic delays of the lenses present. Thus, for example, for an afocal system of axial magnification of −1 consisting of two thin lenses, convergent and identical, of focal length f, the pupil delay is 2δ, that is to say β.T.h2/f.
Thin lenses (convergent or divergent) are generally produced from a material such as glass or silica. This is why thin lenses are also called refractive lenses. It is for this reason that the pupil delay and the dispersive term of refractive lenses are marked hereinafter by the index r (δr and βr).
Thus, for a thin lens made from conventional BK7 glass of index n0=1.5068, we have, at a wavelength of 1053 nm (the period T of a wave at 1053 nm being 3.5 fs), nλ=−1.33×10−5 nm−1 and βr=−262 cm−1.
For a thin silica lens of index n0=1.45 used at the same wavelength, we have nλ=−1.22×10−5 nm−1 and βr=−271 cm−1.
The components that use the diffraction of a grating for ensuring the convergence or divergence of the light beams are called diffractive lenses. Diffractive lenses are for example produced by etching a given density of lines on the face of a diopter. If this etching is carried out on a face of a plate with flat parallel faces, the pupil delay and the dispersive term of these diffractive lenses are referenced by the index d and are as follows:δd=βd.T.(h2/2f) with βd=1/λo 
We find that the dispersive term βd depends only on the wavelength and that its sign is the reverse of that of a diffractive lens.
If this etching is carried out on one of the faces of a refractive lens then the pupil delay is written as the sum of the pupil delays of the refractive and diffractive parts respectively. We will show subsequently that the refractive contribution is in practice negligible and the pupil delay is written as before.
Researchers have used various solutions for compensating for the pupil delay. These solutions are those normally used for the correction of longitudinal chromatism, that is to say for example the replacement of lenses with chromatic doublets, the insertion of a grating the dispersion of which is the reverse of that of a refractive element, etc.
However, recent work has demonstrated that, for use in a power laser with a high delay to be compensated for (that is to say a few ps), only diffractive solutions are viable. See, N. Blanchot, J, Neauport, C. Rouyer et C. Sauteret, Correction of chromatism in the PETAL Chain, July 2006. Solutions for compensating for pupil delay are therefore still restricted.
The main drawback of current solutions is their static character. This is because, once the delay to be compensated for has been evaluated and the correction system has been sized and installed in the optical system, the correction is fixed. Thus, if there is still a delay to be compensated for, whether it be due to an imperfection in the previous evaluation or the manufacture of the compensating element, or it be due to a modification to the optical scheme of the optical system to be compensated (for example a laser chain), the residual delay can be corrected only by a change in the compensation optical element or elements.
The aim of the invention therefore consists of providing a method for adjusting the pupil compensation of a convergent or divergent beam.